The invention relates to compositions, methods, and apparatuses for improving paper surface strength. Paper is sheet material containing interconnected small, discrete fibers. The fibers are usually formed into a sheet on a fine screen from a dilute water suspension or slurry. Paper typically is made from cellulose fibers, although occasionally synthetic fibers are used.
As described in U.S. Pat. No. 5,585,456, paper products made from untreated cellulose fibers lose their strength rapidly when they become wet, i.e., they have very little wet strength. The wet strength of paper is defined as the resistance of the paper to rupture or disintegration when it is wetted with water. Wet strength of ordinary paper is only about 5% of its dry strength. To overcome this disadvantage, various methods of treating paper products have been employed.
One method of increasing the strength of paper is by the addition of a starch coating to the surface of paper. As described in U.S. Pat. No. 4,966,652, although originally applied to size (make resistant to water penetration) paper, starch coatings also increase the stiffness of paper. The increase in stiffness is so pronounced that it makes paper suitable for use in such applications as container board, packaging papers, and sheet fed printer papers. The starch is commonly added onto the paper sheet by an Can-machine process (such as a size press device) or an off-machine process.
As described for example in U.S. patent application Ser. No. 12/323,976, the high cost of paper fiber makes the strength enhancing process even more crucial. Increasingly paper manufacturers are adding significant amounts of less expensive filler materials to defray costs and to enhance other properties required in the paper such as whiteness and brightness. However, papermakers are limited in the amount of fillers in the final product due in great part to a net loss in strength. Tensile strength, z-directional tensile strength and the tendency of the paper to shed filler particles (dusting) during typical handling processes, e.g., printing, are some of the main properties affected. U.S. Pat. No. 7,488,403 describes a method of enhancing the strengthening effect by adding a glyoxylated polyacrylamide polymer to the paper sheet. However there remains a continuing need in the art for methods of imparting appropriate levels of wet strength to paper products.
The art described in this section is not intended to constitute an admission that any patent, publication or other information referred to herein is “prior art” with respect to this invention, unless specifically designated as such. In addition, this section should not be construed to mean that a search has been made or that no other pertinent information as defined in 37 CFR §1.56(a) exists.